The tryptophan synthase Alpha2Beta2 complex is a good model for other multienzyme complexes, including hormone receptors and complexes which synthesize DNA or fatty acids. We are investigating the structure of the Alpha2Beta2 complex and of the component Alpha and Beta2 subunits from 3 different bacterial sources (Escherichia coli, Salmonella typhimurium, and an interspecies hybrid) using studies of solubility, folding properties, domain structure, and susceptibility to limited proteolysis. These comparative studies are aimed at understanding the effects of the divergent evolution of E. coli and S. typhimurium upon the structure and function of enzymes. Most recently we have initiated X-ray crystallographic studies of the Alpha2Beta2 comlex from S. typhimurium which should lead to a much greater understanding of the interaction between the Alpha and Beta2 subunits and of their separate active sites. We also studying the function of tryptophan synthase using chemical modification and investigations with a variety of substrates and substrate analogs. An arginyl residue in the Alpha subunit is modified by phenylglyoxal and protected from modification by the substrate, indole-3-glycerol phosphate. We have discovered that phenylgyoxal is not specific for arginine, as has been generally thought, but also reacts with some sulfhydryl residues in proteins. Our finding that two tryptophan analogs, oxindolyl-L-alanine and 2,3-dihydro-L-tryptophan, are potent competitive inhibitors of both tryptophan synthase and tryptophanase is evidence that the indolenine tautomer of L-tryptophan, which resembles the inhibitors in its geometry at C-3 of the heterocyclic ring, is an intermediate in the reactions catalyzed by both enzymes. 6-Azido-L-tryptophan is an effective photoaffinity inhibitor. Studies of the substrate and reaction specificity of tryptophan show that the enzyme can synthesize a number of S-substituted L-cysteine, S-substituted Beta-methylcysteine, and Se-substituted L-selenocysteine derivatives by Beta-replacement reactions.